Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. PA1 2. Image-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element. PA1 3. Thermal dye transfer printing wherein a visible image pastern is formed by transfer of a coloured species from an image-wise heated donor element onto a receptor element. PA1 MMA=methyl methacylate; IA=itaconic acid; methacrylic acid; and AA=acrylic acid.
Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced. According to U.S. Pat. No. 3,080,254 a typical heat-sensitive copy paper includes in the heat-sensitive layer a thermoplastic binder, e.g ethyl cellulose, a water-insoluble silver salt, e.g. silver stearate and an appropriate organic reducing agent, of which 4-methoxy-1-hydroxy-dihydronaphthalene is a representative.
EP-A 248 405 discloses a thermal recording material with a colour-developing layer, which contains an electron acceptor and an electron donor in addition to the usual additives, characterized in that the electron acceptor is a metallic double salt of a long-chain fatty acid with 16 to 35 carbon atoms.
EP-A 599 580 discloses a thermal recording sheet comprising, in order: (a) a substrate; (b) an intermediate layer which comprises a pigment having an oil absorption according to Japanese Industrial Standard (JIS) K501 of 100 mL/100 g or less; and (c) a thermal color developing layer which comprises a leuco dye type chromogenic component consisting of a leuco dye and an organic color developer and a metal chelate type chromogenic component consisting of an electron acceptor and an electron donor, wherein: the organic color developer is at least one of compounds of formula (I) and formula (II): ##STR2## wherein R is propyl, isopropyl, or butyl; ##STR3## the electron acceptor is a metal double salt of a fatty acid having 16 to 35 carbon atoms; and the electron donor is a polyhydric aromatic compound of formula (III): ##STR4## wherein R is a C.sub.18 - to C.sub.35 -alkyl group, ##STR5## wherein R.sub.1 is a C.sub.18 - to C.sub.35 -alkyl group, n is an integer of 2 or 3, --X-- is --CH.sub.2 --, --CO.sub.2 --, --CO--, --O--, --CONH--, --CO(R')N-- (wherein R' is a C.sub.18 - to C.sub.35 -group), --SO.sub.2 --, --SO.sub.3 -- or --SO.sub.2 NH--.
U.S. Pat. No. 5,582,953 discloses a direct recording process wherein a direct thermal recording material is heated dot-wise and the direct thermal recording material comprises on a substrate an imaging layer, the imaging layer containing uniformly distributed in a film-forming polymeric binder (i) one or more substantially light-insensitive organic silver salts being no double salts, the silver salt(s) being in thermal working relationship with (ii) an organic reducing agent therefor, wherein the reducing agent is a benzene compound the benzene nucleus of which is substituted with a substituent linked to the benzene nucleus by means of a carbonyl group, wherein the carbonyl group is linked to the benzene nucleus at a position which is designated the 1-position of the benzene nucleus, wherein the benzene nucleus is further substituted in a 3-position relative to the 1-position with a single hydroxy group and in a 4-position relative to the 1-position with a single hydroxy group, wherein the reducing agent is selected from the group consisting of an alkyl or aryl ester of 3,4-dihydroxybenzoic acid, 3,4-dihydroxy-benzaldehyde, 3,4-dihydroxy-benzamide and an alkyl or aryl (3,4-dihydroxyphenyl)ketone.
However, prints produced with direct thermal recording materials using reducing agents according to the teaching of U.S. Pat. No. 5,582,953 exhibit unsatisfactory archivability, insufficient light stability over the whole sensitometric range and unsatisfactory colour neutrality. There is therefore a need for direct thermal recording materials produced by coating from solvent or aqueous media, whose prints exhibit improved archivability, improved light stability over the whole sensitometric range and improved colour neutrality.